1. Field of the Invention
The present invention relates generally to a rotary bed sorption system, and, in particular, to a rotary bed sorption system that includes one or more recycled isolation loops. The invention also relates to methods of designing and operating such a system.
2. Description of the Related Art
Rotary bed sorption systems have long been used to collect a sorbate from one fluid stream, sometimes called a process or sorption fluid stream, and transfer it, in a more concentrated form, to a second fluid stream, sometimes called a desorption or regeneration fluid stream. Commonly-removed sorbates include water vapor, volatile organic compounds (“VOCs”), nitrous oxides (“NOx”), and the like.
It is important in a rotary bed sorption system, particularly one in which a low sorbate concentration is desired in the treated fluid, to minimize any cross-contamination between the process fluid stream and the regeneration fluid stream. Contamination can result from pressure differences in the fluid streams themselves, or vapor pressure differences within the matrix of the rotating sorbent disk.
Another source of possible contamination lies in areas of the sorbent disk that are adjacent to the active sorptive area, but that do not see active fluid flow during the cycle. Usually these are the areas at the center and outer periphery of the sorbent disk that are covered by sealing systems. Typically, sorbent disks are intentionally designed to have these “dead zones,” which act as a thermal insulator, keeping the rotor housing cool and preventing its thermal expansion which could lead to premature mechanical failure of the component. It has been demonstrated that in systems designed to achieve a very low sorbate concentration in the treated fluid stream, these areas can act as “sinks” for the vapor being removed. The sinks accumulate vapor when they are adjacent to the regeneration zone, where they are exposed through diffusive forces to the high concentration of the vapors in the regeneration zone, but are not subject to the heat of the regeneration, due to the high insulating characteristics of the glass/ceramic structure. The sorbent then can come to near equilibrium with the high-concentration vapor, but at a much lower temperature. The sinks release their payload when they are adjacent to the very low-concentration fluid in the process section of the system. The vapor pressure difference overcomes the diffusive force of the structure, and trace amounts of vapor are released into the treated fluid stream, causing detrimental increases in the concentration of the vapor being removed.
It is known to incorporate purge zones in rotary bed sorption systems for purposes of cooling the sorbent matrix, recovering regeneration heat, or preconditioning the regeneration fluid stream prior to its passing through the sorbent matrix. An example of a sorption bed system including a recirculating purge loop is disclosed in U.S. Pat. No. 4,701,189. However, there are believed to be no teachings in the prior art of the use of one or more zones to reduce the diffusion of moisture through the sorbent matrix from a higher-concentration fluid stream, or to reduce the carryover of vapor from a higher-pressure fluid stream to a lower-pressure fluid stream.